A necrotic core formed in an advanced atherosclerosis (AS) lesion is a collection of necrotic macrophages (M?) surrounded by inflammatory cells, which contributes to inflammation, plaque disruption, and thrombosis, and eventually develops into a vulnerable plaque. Ordinarily, rapid phagocytic clearance (efferocytosis) by macrophages prevents necrotic core formation, but increasing evidence indicates that efferocytosis is defective in advanced lesions. Although efferocytosis can be up-regulated by HMG Co-A reductase inhibitors (statins)1-3, the comprehensive regulatory mechanism for efferocytosis remains unclear. In the study, we show that statins/ACs treatment activates ERK5 kinase in M? and accelerates efferocytosis via secretion of opsonins and M2 M? polarization, whereas these responses are inhibited in M? derived from macrophage specific ERK5 knock-out (ERK5- MKO) mice. Indeed, ERK5-MKO/LDLR-/- mice showed increased necrotic core and AS formation. On the other hand, p90RSK activated by angiotensin II (AngII) inhibited the ERK5 role in M? functions by inhibiting ERK5 transcriptional activity via ERK5 S496 phosphorylation. In addition, M?-specific wild type-p90RSK transgenic (WT-p90RSK-MTg) crossing to LDLR-/- mice exhibited accelerated formation of necrotic core and AC formation. It suggests that the interplay between p90RSK and ERK5 are critical for the regulation of various M? functions. We therefore hypothesize that statin/ACs-mediated ERK5 kinase activation is a key step in efferocytosis, which inhibits AS and necrotic core formation, while p90RSK activation under AngII, or in advanced atherosclerotic lesions, inhibits efferocytosis via inhibiting ERK5. We will design experiments to address this hypothesis using an in vivo and in vitro approach. The three specific aims of this proposal are 1) We will test the hypothesis that the balance between p90RSK and ERK5 activation under AngII, statins, or CSF-1 stimulation differentially regulates M? functions such as proliferation, apoptosis, M1/M2 phenotypes, and efferocytosis, 2) We will investigate our hypothesis that p90RSK activation induced by AngII or in advanced plaques inhibits ERK5 transcriptional activity and efferocytosis, which promotes the formation of AS plaques and necrotic cores, and 3) We will test the hypothesis that ERK5 activation or inhibition of p90RSK increases efferocytosis and inhibits AS and necrotic core formation via up-regulation of Mfge8. The significance of our study is in providing a mechanistic understanding of the clinically well-described cardiac risk of acute athero-thrombotic vascular disease in advanced atherosclerotic lesions. Also, understanding the role and molecular mechanisms of ERK5- mediated efferocytosis and inhibition of the necrotic core and AS formation should provide insight into the cause of acute atherothrombotic vascular diseases and possibly reveal novel therapeutic targets.